In a word...no.
Desiccated porcine thyroid contains both T3 and T4. In fact, compared to what our thyroids produce, it contains a lot of T3 compared to T4. It's just a different way of delivering T3.
Increasing FT3 does not cause weight gain. Increased FT3 levels raise metabolism and help (allow) you to lose weight. If FT3 and FT4 levels are properly adjusted, weight gain attributable to thyroid is not be an issue.
The enzyme that converts T4 to T3 is a selenium-based enzyme. If you don't think you're converting properly, you might have that checked. Otherwise, so little is known about the process of conversion that the only option is to take T3 if you need it.
Thanks! Your information was really helpful and I appreciate the quick response.